1. Field of the Invention
This invention pertains to color image processing and, more particularly, to a process for preparing color separation images from an original color image wherein highly saturated portions of the original color image are reinforced by a subset of the color separation images.
2. Description of the Prior Art
In conventional processes for reproducing color images, an original color image is electronically scanned and divided into a plurality of areally arrayed picture elements, called pixels. During the scanning operation, it is conventional to use red (R), green (G), and blue (B) filters to intercept a scanning light beam so as to separate the image into its RGB components. A digital number representing the average analog density of the corresponding area on the physical image within that pixel is assigned to each pixel for each of the three color components and retained in memory in separate color image data files from which an electronic image of the original image can be reconstructed. Because of the digital nature of these files, the reconstructed image can be easily stored, processed and viewed, most commonly on a video monitor using standard image processing techniques.
A popular application of this technology within the graphic arts industry occurs when a color-separated image is reconstructed on a color monitor. By electronically altering the stored color image data files for each of the pixels, it is possible to make desired color enhancements or image alterations--a practice known as electronic color editing. When color editing, an operator is able to compensate for imperfect originals, change colors and intensities to produce a more aesthetically pleasing image, as well as add or delete details of the color image to be printed. After this color image manipulation is accomplished on all the pixels within the image, conventional halftone image films are produced from which printing plates or other elements for conveying the colorant to a suitable substrate, hereinafter called plates, are made. The plates are used to reproduce the desired image on a printing press using well known cyan (C), magenta (M), yellow (Y), and black (K) color printing inks or other colorants.
Prior to printing the color image on a printing device, a color proof is frequently made to confirm the suitability of the enhanced image. Color proofs can be produced in several ways including exposing halftone image films for CMYK and using these halftone image films in non-ink proofing systems to produce corresponding CMYK proofs using colorants other than printing inks, such as colored toners, ink-jet inks or precolored imaging layers. Examples of such non-ink proofing systems include Cromalin.RTM. and WaterProof.RTM. sold under these names by E. I. du Pont de Nemours and Company of Wilmington, Del.
An essential part of this electronic color editing process is establishing an accurate correlation between the original image, the electronic data generated during the color separation scanning step, the image displayed on a color monitor, and the final color image as produced on a color printing device. During this correlation effort, compensations are determined for the type of printing device (for example, sheet-fed, offset lithography or gravure) and printing inks used for the display characteristics of the color monitor and, more importantly, for the apparatus (frequently a color scanner with color filters) used to produce input RGB image data files from an original image. During this compensation or calibration process, it is generally the case that the magnitude of color image processing enhancements that can be achieved within an image are limited by the color output range of the printing device to be employed using the traditional four-color-process CMYK printing inks, each ink being associated with a single printing plate. In color image processing, range is defined as the plurality of different color hues and densities controllably reproducible with a predefined set of inks or additional special colorants. A maximum color density value is assigned to each of CMYK based on the maximum amount of ink that can be transferred in a controlled manner from the printing device to the paper being printed upon. Thus, the ability to accurately reproduce an image may be limited to only those densities equal to or less than the print densities available on the printing device. Conventionally, each ink is printed only once during the printing process, i.e., there is only one plate associated with each ink.
Efforts have been made to extend beyond conventional processes by color separating into more than the traditional four-color-process CMYK inks, for instance, by separating into a special set of eight ink colors (CMYKRGB plus white). A printing process employing the special eight printing inks, including an opaque white when printing on a non-white surface, has been introduced by Kuppers, a printing firm in Germany. The process is based on a reference color atlas which provides correlation of printed colors with the required color ink combinations. Also, it is necessary to print all of the seven or eight inks once each to accurately reproduce an image. A major problem with this "CMYKRGB" separation technique is that the halftone image patterns for the extra RGB plates must be made at three new angles in order to avoid moire interference with the halftone image patterns for the CMYK plates. This is a problem because of the well-known need to separate each plate's screen angle from it's nearest neighbors by a multiple of 120.degree., which limits the available number of optimum screen angles to three. To solve this problem, new halftone image printing techniques, such as frequency modulation or stochastic screening, have been employed. These new techniques, in turn, cause other problems such as a coarse image texture and difficulty resolving the micro grain elements on offset plates or proofing materials. These problems, in turn, lead to difficulty providing pre-press proofs that match the color and tonality of a printed sheet. Other problems with the CMYKRGB approach to HiFi color separation include the need for special RGB inks and special proofing materials, and the need to always run seven plates without the economic options to use only five or six plates.
Another similar practice within the industry employs four-color-process CMYK inks to print an image, but overprints that four-color printed image with special RGB color inks to achieve an extended printed color gamut. It is frequently necessary to print each of the seven inks, including the three special RGB inks, to accurately reproduce an image but, as with the so-called Kuppers technique, potential problems with moire interferences and proofing are created.
Another approach to extend the printed color gamut involves the use of "touch plates" which are printing plates that apply a small amount of one or more special color inks to manually selected areas of an image that has already been printed using four-color-process CMYK inks. Such touch plates are generally employed to enhance the appearance of important subjects or to create special color effects that cannot be achieved with CMYK inks alone, but their creation is costly, time consuming and requires great manual skill and experimentation. Prepress proofing for touch plates is generally restricted to the expensive process of "press-proofing" using actual trial plates and the selected color inks. The problem of screen angles mentioned before is also present with touch plates.
U.S. Pat. No. 5,184,214 (Tatsumi) discloses a system for processing an image input signal and producing an image output signal in order to record an image on an image recording medium. The image input signal is converted to a signal which matches the image output system and is also based on corrective parameters to achieve given color qualities. The image signal is also converted to a signal which matches the image recording medium.
U.S. Pat. No. 5,211,546 (Arazi et al.) discloses a method for adjusting color images in which a first image known to print in an acceptable fashion is displayed on a monitoring device, and a second image is displayed on a second device and modified to approximate the visual impression of the first image. In this method, whenever colors are called for that exceed the printing device's color printing range, the operator is alerted to make changes necessary to bring the image back into the available printing range.
U.S. Pat. No. 4,837,613 (Paxton et al.) discloses a method for selecting the intensity level of the primary colors used in displaying or printing a desired color composed of a specified percent of each of the primary colors. Each primary color is represented by two discrete intensity levels for that color. One intensity level is below or equal to the desired color intensity level, and the other is above or equal to the specified percentage.
U.S. Pat. No. 4,992,862 (Gabor) discloses an apparatus for video display that processes printing data in a two-step process. The first step relates the printing data to only one of a plurality of non-additive color components with a selected color reproduction function, and the second step relates the printing data to only two of the plurality of non-additive color components with a selected color reproduction function.
U.S. Pat. No. 5,077,604 (Kivolowitz et al.) describes a method and apparatus for receiving red, green and blue color separation signals from a color scanner and converting these signals into cyan, yellow, magenta and black signals having approximately the same color ratio as the red, green and blue signals. The cyan, yellow, magenta and black signals are subsequently used on a four-color printing device such that the printed image will have approximately the same color ratio as the original red, green and blue image.
U.S. Pat. No. 4,965,664 (Udagawa et al.) describes a color image signal processing apparatus for recording a color image from color image input signals. The apparatus includes a color correction unit for performing color correction based upon a luminance signal converted by a gradation converter, a hue signal and a chroma signal. The hue and chroma signals are based on spectrum tristimulus values which are obtained through conversion of the color image input signals.
In these conventional processes, special efforts are made to ensure that the color range of the image as displayed on the display device is restricted to those colors within the color printing range of the printing device, the printing device itself generally being used to print with four plates, one plate for each of the conventional four-color-process CMYK inks. It is also generally the case that the additive RGB color system employed on the display device is capable of displaying a greater color range than can be printed with conventional four-color-process printing. If it were possible to increase the color printing range of the printing device, it would be possible to increase the corresponding color range of the display device. This would be particularly important when electronic color editing of an image is done to achieve special aesthetic or artistic effects, and also when attempting to match a particular original image. Accordingly, it is believed to be advantageous to provide a method for extending the color printing density range of a printing device without introducing special or non-process printing inks or unconventional pre-press proofing systems, and without introducing additional moire problems or requiring exotic screening methods, while at the same time increasing the printing density range in a manner that is flexible in the number of extra plates that can be used.